The present invention relates to cellular glass structures and in particular to cellular glass structures composed of silicon, oxygen, and carbon made from foamed silicone resins.
Amorphous silica is a refractory glass, however, it devitrifies readily at temperatures greater than 1100.degree. C. Devitrification refers to the ordering or crystallization of the random structures that glasses are made of. Crystallization drastically reduces one of vitreous silicas predominant attributes, i.e., its low thermal expansion, as well as many of its other desirable properties. As a result, much research has been directed to seeking ways to increase the resistance to devitrification in silica glass compositions.
Reactions between silicon, carbon, and oxygen have been studied extensively. Some of the known reactions in a silicon, carbon, and oxygen system include oxygen combining with silicon to form silica, SiO.sub.2. Then at temperatures in excess of 1100.degree. C. silica begins to crystallize to form cristobalite. Cristobalite is one of the common mineral forms of silica. Carbon can react with available silica to form crystalline silicon carbide or escape as carbon monoxide gas. Any carbon remaining as elemental carbon readily oxidizes above 600.degree. C. when exposed to air.
The thermodynamics of silicon, carbon and oxygen reactions is discussed in "The High-Temperature Oxidation, Reduction, and Volatilization Reactions of Silicon and Silicon Carbide", Gulbransen, E. A., and Jansson, S. A. Oxidation of Metals, Volume 4, Number 3, 1972. The thermodynamic analysis of Gulbransen et al. shows that at 1200.degree. C. silica and carbon should form gaseous silicon monoxide and carbon monoxide or solid silicon carbide, SiC. However, no material containing silicon, oxygen and carbon would be expected to form. Gulbransen et al. conclude that silica was not recommended for use in reducing atmospheres above 1125.degree. C. due to the formation of volatile silicon monoxide gas. Also, silicon carbide was not recommended for use in oxygen containing environments where active oxidation may occur due to oxidation of the silicon carbide.
There is a material functionally described as carbon modified vitreous silica and herein referred to as "black glass" where 1-3 percent carbon has been added to silica. The method for making black glass is disclosed by Smith et al. in U.S. Pat. No. 3,378,431. Carbonaceous organics such as carbowax are added to silica and the mixture is hot pressed at about 1200.degree. C. to form black glass. Smith, C. F., Jr. has further characterized black glass by infrared spectroscopy in "The Vibrational Spectra of High Purity and Chemically Substituted Vitreous Silicas", PhD Thesis, Alfred University, Alfred, N.Y., May 1973. Smith discloses that, in addition to elemental carbon dispersed in the glass, carbon in black glass is associated with oxygen in carbonato type groups. A carbonato group is the description of a particular way that a carbon atom bonds with three oxygen atoms and has the structure, ##STR1##
The mechanical strength of black glass is similar to the strength of carbon-free silica glass, however black glass has an increased resistance to devitrification over conventional silica glass which begins to devitrify at about 1100.degree. C., while black glass begins to devitrify at about 1250.degree. C. The increased thermal stability of black glass allows it to be used at temperatures higher than vitreous silica can withstand.
In a commercially produced continuous silicon carbide ceramic fibre sold under the trademark "Nicalon", about 10 percent oxygen is introduced into the fibre to crosslink it. After crosslinking, the fibres are pyrolized and it is believed that the oxygen becomes part of the fibre as an amorphous contaminant, probably in the form of silica. The degradation behavior of such fibres after heat treatment in various environments was reported in the article "Thermal Stability of SiC Fibres (Nicalon.RTM.)", Mah, T., et al., Journal of Material Science, Vol. 19, pp. 1191-1201 (1984). Mah et al. found that regardless of the environmental conditions during heat treatment, the "Nicalon" fibre strength degraded when the fibres were subjected to temperatures greater than 1200.degree. C. The fibre degradation was associated with loss of carbon monoxide from the fibres and beta-silicon carbide grain growth in the fibres.
From the discussion above, it is apparent that the properties of known ceramic or glass compositions, and specifically those containing silicon, oxygen and carbon, are degraded by decomposition or devitrification of the glass or ceramic at temperatures above 1100.degree. C. to 1250.degree. C.
Heretofore known methods of forming glass or ceramic materials into cellular glass or ceramic structures utilize glass or ceramic powders or slurries of the powders to form the cellular body. For example, in some methods cellular glass and ceramic structures are formed from glass or mineral powders that are intimately mixed with a gas-producing agent, such as carbon black. The mixture is heated to a temperature sufficient to sinter and cohere the particles of glass or mineral and simultaneously liberate bubbles of gas in relatively uniform distribution in the sintered mass.
Another method for forming cellular ceramic structures is disclosed in U.S. Pat. No. 3,833,386. An isocyanate capped polyoxyethylene polyol is reacted with large amounts of an aqueous slurry of sinterable ceramic material. A hydrophilic crosslinked polyurethane foam is generated having sinterable material uniformly disposed throughout. The foamed structure is heated in an atmosphere of air, oxygen, inert gases or the like to decompose the polyurethane and sinter the remaining sinterable ceramic material forming a rigid ceramic foam structure.
An important property of cellular glass structures is the degree to which the cell walls are open or closed. Open cell walls provide intercommunication between cells so that the cellular body can act as a filter or sound absorbing structure. With closed cell walls there is no intercommunication between cells and the cellular body has better insulating properties. One method for providing open cell walls in sintered cellular glass structures is disclosed in U.S. Pat. No. 2,596,659. A closed cell structure is subjected to fluid pressure and the thin walls of the cells at their weakest points tend to rupture in a manner that provides communication between cells.
The durability of prior art cellular glass structures in certain agencies, such as water at elevated temperatures, was not all that might be desired because the alkali content tended to leach out, causing disintegration or deterioration of the cellular glass.
Sintered ceramic bodies inherently contain up to 10 percent porosity, which porosity reduces the strength of the ceramic body. One method of reducing the porosity and improving the strength of sintered ceramics is to re-infiltrate the ceramic body with the powdered ceramic and sinter the reinfiltrated ceramic. This can be repeated many times to continue filling matrix porosity. However, such methods of filling porosity in ceramic bodies would be difficult to perform in sintered cellular ceramic structures. Such re-infiltration would fill the cells in cellular structures having open cell walls and would fill the porosity on the outer surface of cellular structures having closed cell walls leaving porous cell walls inside the structure unaffected.
Sintered ceramics are also known, to exhibit grain boundaries that block the mass transport of second phase particles causing an uneven distribution of the second phase particles in the ceramic body.
Therefore, it is an object of this invention to form a cellular glass structure, comprising silicon, oxygen and carbon wherein a substantial portion of the carbon atoms are bonded to silicon atoms and the remaining carbon is elemental carbon dispersed in the glass matrix. Such cellular glass structures remain structurally stable and do not decompose in oxidizing or reducing atmospheres at temperatures up to at least 1650.degree. C.
Another object of this invention is a process for forming cellular glass structures comprised of silicon, oxygen and carbon by pyrolizing foamed methyl silicone resins.
Another object of this invention is a process for forming cellular glass structures comprised of silicon, oxygen and carbon where the cell walls are essentially fully dense, amorphous, and free of grain boundaries.